Unsaturated thermosetting polyester resins are widely used in industry today for the production of moldings, for example, automobile parts molded from sheet-molding compound, electical breaker boxes, appliance housings, business machine housings, and a wide range of reinforced parts. These resins are polycondensation products which are based on the reaction of dibasic acids with dihydric alcohols. Ethylenic unsaturation is incorporated into the resin, generally by use of an unsaturated acid (e.g., maleic anhydride); although a saturated acid, (e.g. isophthalic or adipic acid) may also be present. The alcohol is typically saturated, e.g., propylene glycol. The unsaturated polyesters are usually crosslinked through their double bonds with a compatible monomer (such as styrene) which also contains ethylenic unsaturation, and thereby become thermoset. The polyester itself is often blended with a thermoplastic oligomer in order to obtain modified physical property characteristics. For example, a polyvinyl acetate/styrene oligomer and/or an acrylic oligomer such as a polymethacrylate/styrene oligomer are commonly included as so-called "low-profile additives" to improve surface smoothness of the resulting molded article. (As used herein, the term "polyester resin" is meant to include not only the polyester itself but also the crosslinking monomer which is conventionally added to the polyester and any other modifying resin that is blended with the polyester). The molding material may also typically include a conventional filler and/or reinforcing agent, a mold release agent and a thickener.
To effect crosslinking of the polyester, a catalyst of the organic peroxide type is generally used. The catalyst is generally premixed with the resin prior to molding and is thermally activated upon molding to initiate the crosslinking reaction. Using such a technique, curing does not begin until the mixture containing the catalyst is exposed to activating temperatures typically in the range of 275.degree. to 350.degree. F.; the optimum temperature to be used depends upon the specific nature of the catalyst, the duration of the molding cycle, and other factors.
Prior to conducting the sheet molding operation, so-called "sheet molding compound" or "SMC" is conventionally prepared by adding a stream containing a thickener to a stream containing unsaturated polyester and any other modifying resin. The resulting paste is passed through a so-called "SMC machine" to provide a composite sandwich having a middle layer of reinforcing agent, such as chopped fiber glass roving, and a top and bottom layer of this paste. The composite is then pressed by, for example, chain belt compaction rollers at a pressure of between 20 and 80 psi to provide good "wet-out" of the reinforcing agent by the paste and to initiate ionic polymerization of the paste up to a viscosty of about 15 to 20 million centipoise, thus providing a leather-like composite sheet which is ready to be cut into strips for the subsequent sheet molding operation, typically in a heated compression mold.
If a thickener is used in the conventional manner as described above, various disadvantages and obstacles are encountered. First, there are extra equipment costs and potential processing problems associated with the presence of the second stream utilized to provide thickener for the SMC composition. Second, and more importantly, the conventional addition of thickener to form the sheet molding composition sometimes results in the premature thickening of the SMC composition, thereby clogging the SMC machine and causing down-time and associated production losses.
If a thickener is not used in making the composite, there frequently are processing problems on the SMC machine due to loss of paste out the sides of the hot press since, without thickener, the paste has a low viscosity and tends to flow out of the press rather than bind to the reinforcing agent when the press is closed.
In view of the above, it would be highly desirable to provide a composition characterized by a controlled thickening capability. In a sheet molding context, such a composition is expected to be highly desired since it would avoid or minimize the above-discussed SMC extra equipment costs and processing problems.